Updates from NIAC 2016

Action items from minutes completed:
-H,K,L change to NX_number for incommensurate structures
-Discussion on flags, change to bitfield, no enum for efficiency
-Expand the description of partiality
-Need to keep mm and pixel positions because of parallax
-Write out predicted and observed
-Drop the mm in names, rather use units
-bounding_box as array, document the usage
-Spell out background
-Drop the val on the intensities, keep the _var version
code is not in place we defer it for now.
-Add polar_angle and azimuthal_angle

TODO
-Need documentation for polar_angle and azimuthal_angle
-Need to add some examples of flags. Would like to use the DIALS flags.

Deferred:
-overlaps is a list of overlapped reflections. This is a ragged array. As the